Medline (January 1966 to December 2002) and Embase (January 1980 to December 2002) searches were supplemented with material identified in the references and in the authors' personal files. To investigate whether funding of drug studies by the pharmaceutical industry is associated with outcomes that are favourable to the funder and whether the methods of trials funded by pharmaceutical companies differ from the methods in trials with other sources of support. No convincing evidence supports the use of dietary interventions for the prevention of breast cancer, with the exception of limiting alcohol intake. Factors associated with decreased cancer rates include pregnancy at an early age, late menarche, early menopause, high parity, and use of some medications, such as selective estrogen receptor modulators and, possibly, nonsteroidal anti-inflammatory agents and aspirin.
Obesity is also associated with increased rates of breast cancer. Environmental factors include therapeutic radiation. Hormonal factors associated with breast cancer include advanced age at first pregnancy, exposure to diethylstilbestrol, and hormone therapy. Nonmodifiable factors associated with increased breast cancer risk include advanced age, female sex, family history of breast cancer, increased breast density, genetic predisposition, menarche before age 12 years, and natural menopause after age 45 years. Understanding modifiable and nonmodifiable factors that increase or decrease breast cancer risk allows family physicians to counsel women appropriately.
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